Research has focused on determining the genetic and physiological basis of susceptibility or refractoriness to malaria in anopheline mosquitoes. Studies have emphasized defining refractoriness in selected strains of the African malaria vector, Anopheles gambiae. In one strain, refractoriness is manifested by the death and encapsulation of early malarial oocysts on the mosquito gut. Backcross studies indicate that refractoriness is probably controlled by two unlinked loci, one of which has a major effect on refractoriness. The second locus is closely linked to an isoenzyme locus. The precise mode of inheritance of refractoriness is being determined.